There is known an animation plotting method in which an NC machining program simulates a profile of a workpiece viewed from the direction orthogonal to a spindle and the axis of a tool on a graphic display screen of an NC system. FIG. 6 depicts an example where, in turning a workpiece 100 rotated around a horizontal axis Z by a spindle with a cutting tool 101 such as a turning tool mounted along a vertical axis X, the profile of the workpiece is plotted on a graphic display screen 73 in accordance with a conventional animation plotting method.
The movement of the tool 101 during ordinary turning operation is limited to cutting in an X-axis direction and feeding in a Z-axis direction, and hence the change in outer diameter of the workpiece 100 rotating around the Z-axis is always constant. Thus, the NC machining program can be fully checked by merely plotting the turning condition of the workpiece 100 viewed from the direction of a Y-axis orthogonal to the Z-axis and the X-axis as shown in FIG. 6.
Recently, however, as shown in FIG. 5, there has become available an NC lathe having a multiplicity of tool rests, which are not only capable of performing the turning machining but also capable of performing the plane cutting and engraving on the planed outer peripheral surface of the workpiece 100 by controlling the position of the spindle for a desired rotational point (C-axis control). In this type of NC lathe, the tool 102 is displaceable in all directions of the X, Y and Z axes, and the machining surface on the workpiece 100 is not necessarily in parallel with the X-Z plane. It is therefore difficult for the conventional plotting method as is shown in FIG. 6, or for the plotting method of plotting the cutting condition of the workpiece 100 viewed from the direction of the Y-axis, to fully check the NC machining program.